I'd take that bullet list backwards:
4. Do nothing, and let these questions languish here.
Certainly not. That doesn't help anyone.
3. Try to entice EE users to try their hand at reviewing vhdl here on CR
With 4 questions in some 18 months since the tag's creation, bringing EE users here is a bit of a stretch. Not impossible, but you'd have to find an EE user that's more into programming, I believe.
2. Ask authors to repost their questions on EE
That sounds more realistic. The question isn't off-topic on CR, it's just that Stack Exchange happens to have a more specialized site where vhdl might have a wider audience. I don't see cross-posting as a problem. Unless...
1. Routinely migrate requests to review vhdl to EE (on grounds that VHDL is off-topic for CR)
Off-topic... but on what grounds? I know nothing about it, but VHDL sure looks like code to me. Not to cite Wikipedia as an authoritative source, but...
Due to the Department of Defense requiring as much of the syntax as possible to be based on Ada, in order to avoid re-inventing concepts that had already been thoroughly tested in the development of Ada, VHDL borrows heavily from the Ada programming language in both concepts and syntax.
Ada is a structured, statically typed, imperative, wide-spectrum, and object-oriented high-level computer programming language, extended from Pascal and other languages.
Looks like a programming language to me, and a useful one on top of it. I think considering it off-topic just because we don't have the expertise is fraying with a slippery slope. Would php be off-topic if there weren't enough reviewers for it?
I vote for #2 (cross-posting), and if people there start sending vhdl askers over here, then they'll
inevitably hopefully bring some reviewers along with these questions. Perhaps some of these reviewers know some c or c++, too. At the end of the day, it's about visibility / getting the right eyes on the piece of code.